How phosphorus helped oxygenate Earth’s atmosphere

The hazy atmosphere of the early Earth during the Archean lacked oxygen, possibly in part because of the slow rate of phosphorus recycling in the oceans. Image: NASA GSFC/Francis Reddy.

Life in Earth’s oceans may have had a slow start because phosphorus – a key nutrient of life – was not recycled through the biosphere fast enough. The finding, by scientists at the University of Washington and the University of St Andrews, UK, could explain why it took so long for Earth’s atmosphere to become oxygenated.

By modeling Earth’s oceans over billions of years, Washington’s Michael Kipp and Eva Stüeken of St Andrews, found that during the Archean (the geological era of between four and 2.5 billion years ago) phosphorus was recycled ten times slower than in the oceans today, resulting in biological productivity slowing down.

This has implications for the presence of oxygen in Earth’s atmosphere. During the Archean, the atmosphere was dominated by carbon dioxide. Around two-and-a-half billion years ago, oxygen suddenly filled the atmosphere during what is known as the Great Oxygenation Event. It is believed that photosynthesizing life contributed to this oxygen influx, but the first signs of oxygenic photosynthesis are from three billion years ago. Something delayed the oxygenation of the atmosphere and Kipp and Stüeken suspect that the limited phosphorus recycling in the oceans may have played its part.

Phosphorus, in the form of phosphate, washes into the oceans where it is ingested by plankton and algae, which in turn are consumed by other organisms. The phosphorus cycles through the ocean ecosystem several times before organisms die, sink to the seabed and decay, releasing phosphorus that microbes can utilize to digest food, increasing biological activity and the biological production of oxygen.

For this process to operate efficiently, oxygen is required, but in the Archean there was very little. So life found other ways, in particular using sulfate derived from volcanos instead of using oxygen. However, there was very little sulfate in the Archean ocean, thereby limiting the amount of biomass that could be digested and hence constraining the amount of biologically-produced oxygen in the ocean, which in turn drastically slowed the oxygenation of the atmosphere.

Volcanism could have provided the sulfate that did the job of oxygen in recycling phosphorus. Image: USGS/Hawaiian Volcano Observatory/J. D. Griggs.

This cycle was only broken when sulfate became more available to life in the oceans as a result of increased volcanism. Indeed, without volcanic sulfate to stand in for the lack of oxygen, Earth’s biosphere may have struggled to survive.

“As soon as sulfate became more abundant in the oceans during the late Archean, it is possible that the increased phosphorus recycling would have led to higher productivity and more oxygen production, leading to more sulfate and higher productivity and so on,” says Kipp. It is this positive feedback that could have, over at least half a billion years, eventually contributed to the Great Oxygenation Event.

This also has astrobiological implications. Low phosphorus abundance could make an inhabited planet appear uninhabited, because of the lack of oxygen in the atmosphere, when in reality their biosphere is just too small to create a viable biosignature.